The invention relates to subterranean wells for the injection, storage, or production of fluids. More particularly it relates to plugging fractures in formations in such wells.
Fractures in reservoirs normally have the highest flow capacity of any portion of the reservoir formation. These fractures in the formation may be natural or hydraulically generated. In a natural fault in the rock structure, the high flow capacity results either from the same factors as for natural fractures or from the fracture being open for example due to natural asperities or because the rock is hard and the closure stress is low. In artificially created fractures, such as those created by hydraulic fracturing or acid fracturing, the high flow capacity results from the fracture being either propped with a very permeable bed of material or etched along the fracture face with acid or other material that has dissolved part of the formation.
Fractures of interest in this field are typically connected to the formation and to the wellbore. Large volumes of fluids will travel through fractures due to their high flow capacity. This allows wells to have high fluid rates for production or injection. Normally, this is desirable.
However, in the course of creating or using an oil or gas well, it is often desirable to plug or partially plug a fracture in the rock formations, thereby reducing its flow capacity. Typically the reasons for plugging these fractures are that a) they are producing unwanted water or gas, b) there is non-uniformity of injected fluid (such as water or CO 2) in an enhanced recovery flood, or c) expensive materials (such as hydraulic fracturing fluids during fracturing) are being injected into non-producing areas of the formation. This latter case can be particularly deleterious if it results in undesirable fracture growth because at best it wastes manpower, hydraulic horsepower, and materials, to produce a fracture where it is not needed, and at worst it results in the growth of a fracture into a region from which undesirable fluids, such as water, are produced.
Past techniques for plugging fractures have included cement systems, hydrating clays, and both crosslinked and non-crosslinked polymer solutions. The disadvantages of cement systems are the requirements for expensive materials and well work, and the systems” inability to travel down the fracture without bridging prematurely. The hydrating clays require the complexity and cost of pumping oil-based systems plus expensive well work. The hydrating clays also have the same problem as the cement with regard to placement: needing to avoid premature bridging; they also have the requirement of needing to hydrate fully along the fracture. The polymer systems often fail due to their lack of flow resistance in very permeable fractures and because the materials are expensive considering the large volumes that are required. There is a need for an inexpensive, reliable, easily placed, effective well plugging material and methods for use during well completion or remediation, especially stimulation, and during fluids production.